The present invention relates to a film producing method used to produce a film or sheet of a cellulose ester such as cellulose triacetate (TAC), cellulose diacetate, or the like.
In a conventional method for producing such a film, a film-forming solution is prepared by dissolving a cellulose ester such as cellulose triacetate, cellulose diacetate, or the like, to which is added a small quantity of plasticizer, in an organic solvent containing methylene chloride as a main component. As shown in FIG. 4, the film-forming solution is made to flow over a polished metal support 3 by passing the film-forming solution through a slot 2 of an extrusion die 1.
In this process, however, a slug 5 of cellulose acetate tends to format the lip 4 of the outlet slot 2 of the die 1 at the two end portions of the slot 2 due to evaporation of the solvent contained in the film-forming solution after the process has been carried out for a long period. This is termed "slugging". When such slug 5 grows, there has been a problem in that the slug 5 interferes with the flow of the film-forming solution emerging from the outlet slot 2, thereby resulting in the formation of a film 17 having uneven edge portions 6. Also, sometimes the slug 5 separates from the lip 4, allowing the separated slug 5 to damage the film, for example, when the separated slug 5 adheres to a feed roller in a subsequent stage.
In order to prevent such slugging from occurring, a method in which a gas mixture of an evaporated solvent and air is sprayed in the vicinity of the opposite ends of the slot of the extrusion die has been used. A method is also known in which a solvent is dropped onto the film-forming solution, for example, at a flow rate 0.3 to 5 cc/min, at the opposite end portions of the extrusion slot. Such methods are disclosed, for example, in U.S. Pat. No. 3,112,528 and Japanese Patent Unexamined Publication No. Hei. 2-208650.
In the former method in which a gas mixture is sprayed, however, the flow of the gas can sometimes cause an unevenness in thickness of the formed film. In the latter method, although slugging can be prevented, the concentration of the solvent in the base becomes high, as a result of which the drying of the film on the support becomes slow at portions (base end portions) where the solvent is dropped onto the film-forming solution because the film absorbs the dropped liquid. As a result, incomplete separation of the film from the support occurs. To overcome this difficulty, it has been required to periodically clean the support or reduce the rate of film formation so as to increase the drying time on the support. Such measures though have a disadvantage in that they inevitably result in a lowered production efficiency.